The present invention relates to a welding power supply unit for applying a welding current to materials to be welded to generate Joule heat to weld the materials.
In the case where, as in a seam welding operation, welding current is applied to materials to be welded to generate Joule heat to weld the materials, the amount of Joule heat produced is represented by I.sup.2 Rt, that is, it is proportional to the square of the welding current. Therefore, in a welding operation with a sinusoidal current of a commercial frequency 50 Hz or 60 Hz, the amount of heat generated varies greatly, with the result that sputtering, expulsion and surface flashing often occur, welded portions crack, and the welding quality is lowered.
In a thin plate welding operation, since the materials to be welded have small thermal capacities, it is considerably difficult to determine suitable welding conditions and to perform material control for the materials to be welded.
If a welding machine having a low load power factor is employed in a seam welding operation carried out with single-phase, square-wave voltage, a welding current having a substantially triangular waveform flows. Therefore, the seam welding operation suffers from difficulties similar to those which have been described above.
In order to eliminate these difficulties, Japanese Laid-Open Patent Application No. 25886/1983 has proposed a welding method in which, when the welding current exceeds a predetermined value, a switching element in a power inverter provided for an AC power source is intermittently operated to obtain a square waveform welding current. However, this conventional technique is disadvantageous in that, as it is necessary to turn on and off the switching element at high speed, the power loss is relatively high and the switching element must be large in capacity. In addition, turning on and off the switching element at high speed increases the reactive power.